K&N 93-97 Jaguar XJ12 drop in replacement air filters are awesome. Not only are they easy to install (just pop off the clips on your XJ12 stock air box, grab your old filter, and drop your new K&N air filter in), but they are economical AND effective.
Your stock XJ12 Jaguar paper air filter is not a good air filter. Not only do paper air filters have to be replaced fairly often, but as dirt and dust accumulates on top of them, they clog and lose their ability to filter air properly. The result is reduced performance and increased strain on your air-hungry XJ12 motor. K&N air filters on the other hand, are made from pleated cotton with microfine fibers, and oiled to perfection, resulting in an air filter that actually works better as it gets dirtier (to a point of course). Also, since this Jaguar XJ12 air filter is reusable, you only need to clean it with K&N oil every 50,000 miles, instead of replacing it every 1-2 years like you are supposed to with your stock Jaguar XJ12 drop-in filter.
Guaranteed for a million miles and 50 state emissions legal.